Prevent updating invlid child task surfaces in the sync queue

The task surface might be released before running in the sync queue for
the case like trampoline launch, so check if the surface is valid before
processing it in StageTaskListener.

Fix: 239490752
Test: atest WMShellUnitTests
Test: no crash when dragging a "clean" shortcut of Files to enter split
Change-Id: I8f927f644149cf9b24f4c9110f8ac745bb558dcd